Yeast killer systems

Antimicrobial activity of immobilized mycocins in sodium alginate on fecal coliforms

Wickerhamomyces anomalus is a yeast-producing mycocins and has antimicrobial action. Escherichia coli is the predominant bacterium of the coliforms group; its presence in water indicates fecal contamination, being used as an indicator of microbiological analyses. The immobilization of cells and substances demonstrates great potential for biotechnological applications. This work aimed to assess the activity of free and immobilized mycocins, obtained from W. anomalus, against E. coli strains and fecal coliforms. The mycocins were immobilized in different concentrations of sodium alginate and calcium chloride and tested to verify the antimicrobial activity against the E. coli strain and fecal coliforms present in water samples. The mycocins were able to inhibit all strains used in broth microdilution. Considering the problem of multidrug-resistant antibiotic strains and the need for new alternatives to improve the quality of water and sewage effluents, these results demonstrate a possible application as an alternative to an antimicrobial agent.

The Killer Saccharomyces cerevisiae Toxin: From Origin to Biomedical Research

The killer systems of S. cerevisiae are defined by the co-infection of two viral agents, an M virus and a helper virus. Each killer toxin is determined by the type of M virus (ScV-M1, ScV-M2, ScV-M28, and ScV-Mlus), which encodes a specific toxin (K1, K2, K28, and Klus). Since their discovery, interest in their potential use as antimicrobial agents has driven research into the mechanisms of action of these toxins on susceptible cells. This review provides an overview of the key aspects of killer toxins, including their origin and the evolutionary implications surrounding the viruses involved in the killer system, as well as their potential applications in the biomedical field and as a biological control strategy. Special attention is given to the mechanisms of action described to date for the various S. cerevisiae killer toxins.

The Prevalence of Killer Yeasts in the Gardens of Fungus-Growing Ants and the Discovery of Novel Killer Toxin named Ksino

Killer toxins are proteinaceous antifungal molecules produced by yeasts, with activity against a wide range of human and plant pathogenic fungi. Fungus gardens of attine ants in Brazil were surveyed to determine the presence of killer toxin-producing yeasts and to define their antifungal activities and ecological importance. Our results indicate that up to 46% of yeasts isolated from specific fungal gardens can be killer yeasts, with an overall prevalence of 17% across all strains tested. Killer yeasts were less likely to inhibit the growth of yeasts isolated from the same environment but more effective at inhibiting yeast isolated from other environments, supporting a role for killer yeasts in shaping community composition. All killer yeasts harbored genome-encoded killer toxins due to the lack of cytoplasmic toxin-encoding elements (i.e., double-stranded RNA satellites and linear double-stranded DNAs). Of all the killer yeasts identified, an isolate of Candida sinolaborantium showed a broad spectrum of antifungal activities against 57% of yeast strains tested for toxin susceptibility. The complete genome sequence of C. sinolaborantium identified a new killer toxin, Ksino, with primary and tertiary structure homology to the Saccharomyces cerevisiae killer toxin named Klus. Genome-encoded homologs of Ksino were found in yeast strains of Saccharomycetes and Pichiomycetes, as well as other species of Ascomycota and Basidiomycota filamentous fungi. This demonstrates that killer yeasts can be widespread in attine ant fungus gardens, possibly influencing fungal community composition and the importance of these complex microbial communities for discovering novel antifungal molecules.

Intestinal fungal-host interactions in promoting and maintaining health

The resident microbiota are a key component of a healthy organism. The vast majority of microbiome studies have focused on bacterial members, which constitute a significant portion of resident microbial biomass. Recent studies have demonstrated how the fungal component of the microbiota, or the mycobiome, influences mammalian biology despite its low abundance compared to other microbes. Fungi are known for their pathogenic potential, yet fungi are also prominent colonizers in healthy states, highlighting their duality. We summarize the characteristics that define the gut mycobiome across life, the factors that can impact its composition, and studies that identify mechanisms of how fungi confer health benefits. The goal of this review is to synthesize our knowledge regarding the composition and function of a healthy mycobiome with a view to inspiring future therapeutic advances.

Bioaugmented ensiling of sweet sorghum with Pichia anomala and cellulase and improved enzymatic hydrolysis of silage via ball milling

Sweet sorghum, as a seasonal energy crop, is rich in cellulose and hemicellulose that can be converted into biofuels. This work aims at investigating the effects of synergistic regulation of Pichia anomala and cellulase on ensiling quality and microbial community of sweet sorghum silages as a storage and pretreatment method. Furthermore, the combined pretreatment effects of ensiling and ball milling on sweet sorghum were evaluated by microstructure change and enzymatic hydrolysis. Based on membership function analysis, the combination of P. anomala and cellulase (PA + CE) significantly improved the silage quality by preserving organic components and promoting fermentation characteristics. The bioaugmented ensiling with PA + CE restructured the bacterial community by facilitating Lactobacillus and inhibiting undesired microorganisms by killer activity of P. anomala. The combined bioaugmented ensiling pretreatment with ball milling significantly increased the enzymatic hydrolysis efficiency (EHE) to 71%, accompanied by the increased specific surface area and decreased pore size/crystallinity of sweet sorghum. Moreover, the EHE after combined pretreatment was increased by 1.37 times compared with raw material. Hence, the combined pretreatment was demonstrated as a novel strategy to effectively enhance enzymatic hydrolysis of sweet sorghum.

RNA viruses, M satellites, chromosomal killer genes, and killer/nonkiller phenotypes in the 100-genomes S. cerevisiae strains

We characterized previously identified RNA viruses (L-A, L-BC, 20S, and 23S), L-A-dependent M satellites (M1, M2, M28, and Mlus), and M satellite-dependent killer phenotypes in the Saccharomyces cerevisiae 100-genomes genetic resource population. L-BC was present in all strains, albeit in 2 distinct levels, L-BChi and L-BClo; the L-BC level is associated with the L-BC genotype. L-BChi, L-A, 20S, 23S, M1, M2, and Mlus (M28 was absent) were in fewer strains than the similarly inherited 2µ plasmid. Novel L-A-dependent phenotypes were identified. Ten M+ strains exhibited M satellite-dependent killing (K+) of at least 1 of the naturally M0 and cured M0 derivatives of the 100-genomes strains; in these M0 strains, sensitivities to K1+, K2+, and K28+ strains varied. Finally, to complement our M satellite-encoded killer toxin analysis, we assembled the chromosomal KHS1 and KHR1 killer genes and used naturally M0 and cured M0 derivatives of the 100-genomes strains to assess and characterize the chromosomal killer phenotypes.

The evolutionary ecology of fungal killer phenotypes

Ecological interactions influence evolutionary dynamics by selecting upon fitness variation within species. Antagonistic interactions often promote genetic and species diversity, despite the inherently suppressive effect they can have on the species experiencing them. A central aim of evolutionary ecology is to understand how diversity is maintained in systems experiencing antagonism. In this review, we address how certain single-celled and dimorphic fungi have evolved allelopathic killer phenotypes that engage in antagonistic interactions. We discuss the evolutionary pathways to the production of lethal toxins, the functions of killer phenotypes and the consequences of competition for toxin producers, their competitors and toxin-encoding endosymbionts. Killer phenotypes are powerful models because many appear to have evolved independently, enabling across-phylogeny comparisons of the origins, functions and consequences of allelopathic antagonism. Killer phenotypes can eliminate host competitors and influence evolutionary dynamics, yet the evolutionary ecology of killer phenotypes remains largely unknown. We discuss what is known and what remains to be ascertained about killer phenotype ecology and evolution, while bringing their model system properties to the reader's attention.

Chitinolytic Enzymes of the Hyperparasite Fungus Aphanocladium album: Genome-Wide Survey and Characterization of A Selected Enzyme

The filamentous fungus Aphanocladium album is known as a hyperparasite of plant pathogenic fungi; hence, it has been studied as a possible agent for plant protection. Chitinases secreted by A. album have proven to be essential for its fungicidal activity. However, no complete analysis of the A. album chitinase assortment has been carried out, nor have any of its chitinases been characterized yet. In this study, we report the first draft assembly of the genome sequence of A. album (strain MX-95). The in silico functional annotation of the genome allowed the identification of 46 genes encoding chitinolytic enzymes of the GH18 (26 genes), GH20 (8 genes), GH75 (8 genes), and GH3 (4 genes) families. The encoded proteins were investigated by comparative and phylogenetic analysis, allowing clustering in different subgroups. A. album chitinases were also characterized according to the presence of different functional protein domains (carbohydrate-binding modules and catalytic domains) providing the first complete description of the chitinase repertoire of A. album. A single chitinase gene was then selected for complete functional characterization. The encoded protein was expressed in the yeast Pichia pastoris, and its activity was assayed under different conditions of temperature and pH and with different substrates. It was found that the enzyme acts mainly as a chitobiosidase, with higher activity in the 37-50 °C range.

Uncovering the Yeast Communities in Fungus-Growing Ant Colonies

Yeast-insect interactions are compelling models to study the evolution, ecology, and diversification of yeasts. Fungus-growing (attine) ants are prominent insects in the Neotropics that evolved an ancient fungiculture of basidiomycete fungi over 55-65 million years, supplying an environment for a hidden yeast diversity. Here we assessed the yeast diversity in the attine ant environment by thoroughly sampling fungus gardens across four out of five ant fungiculture systems: Acromyrmex coronatus and Mycetomoellerius tucumanus standing for leaf-cutting and higher-attine fungicultures, respectively; Apterostigma sp., Mycetophylax sp., and Mycocepurus goeldii as ants from the lower-attine fungiculture. Among the fungus gardens of all fungus-growing ants examined, we found taxonomically unique and diverse microbial yeast communities across the different fungicultures. Ascomycete yeasts were the core taxa in fungus garden samples, with Saccharomycetales as the most frequent order. The genera Aureobasidium, Candida, Papiliotrema, Starmerella, and Sugiyamaella had the highest incidence in fungus gardens. Despite the expected similarity within the same fungiculture system, colonies of the same ant species differed in community structure. Among Saccharomycotina yeasts, few were distinguishable as killer yeasts, with a classical inhibition pattern for the killer phenotype, differing from earlier observations in this environment, which should be further investigated. Yeast mycobiome in fungus gardens is distinct between colonies of the same fungiculture and each ant colony harbors a distinguished and unique yeast community. Fungus gardens of attine ants are emergent environments to study the diversity and ecology of yeasts associated with insects.

Application of Bioactive Coatings with Killer Yeasts to Control Post-Harvest Apple Decay Caused by Botrytis cinerea and Penicillium italicum

A new method was proposed to produce alginate bio-films containing Pichia membranifaciens and Wickerhamomyces anomalus killer yeast to control the post-harvest fungal decay in organic apples caused by Botrytis cinerea and Penicillium italicum. Coatings with W. anomalus killer yeast effectively controlled the growth of P. italicum during storage at 22 °C. W. anomalus killer yeast incorporated in alginate reduced the P. italicum incidence from 90% (control) to 35% after 14 days of storage at 22 °C. Alginate biofilms with W. anomalus or P. membranifaciens also limited the incidence of the fungal decay of apples inoculated with B. cinerea compared with the control fruits, although the antagonistic capability against B. cinerea was lower than against P. italicum. The survival of W. anomalus cells in alginate coating was higher than P. membranifaciens. The incorporation of killer yeasts into alginate had no significant effect on the mechanical properties (tensile strength, percent elongation at break) of alginate coating, however, they increased the thickness of the biofilm. The bioactive coating reduced the fruit weight loss and had no significant effects on the fruit firmness during storage at 2 °C. As organic apples, produced without any synthetic fungicides, are especially prone to fungal decay during storage, the proposed alginate biofilms containing killer yeast seem to be a very promising solution by offering non-chemical, biological control of post-harvest pathogens.

Women with type 1 diabetes exhibit a progressive increase in gut Saccharomyces cerevisiae in pregnancy associated with evidence of gut inflammation

AIMS

Studies of the gut microbiome have focused on its bacterial composition. We aimed to characterize the gut fungal microbiome (mycobiome) across pregnancy in women with and without type 1 diabetes.

METHODS

Faecal samples (n = 162) were collected from 70 pregnant women (45 with and 25 without type 1 diabetes) across all trimesters. Fungi were analysed by internal transcribed spacer 1 amplicon sequencing. Markers of intestinal inflammation (faecal calprotectin) and intestinal epithelial integrity (serum intestinal fatty acid binding protein; I-FABP), and serum antibodies to Saccharomyces cerevisiae (ASCA) were measured.

RESULTS

Women with type 1 diabetes had decreased fungal alpha diversity by the third trimester, associated with an increased abundance of Saccharomyces cerevisiae that was inversely related to the abundance of the anti-inflammatory butyrate-producing bacterium Faecalibacterium prausnitzii. Women with type 1 diabetes had higher concentrations of calprotectin, I-FABP and ASCA.

CONCLUSIONS

Women with type 1 diabetes exhibit a shift in the gut mycobiome across pregnancy associated with evidence of gut inflammation and impaired intestinal barrier function. The relevance of these findings to the higher rate of pregnancy complications in type 1 diabetes warrants further study.

Antagonism between killer yeast strains as an experimental model for biological nucleation dynamics

Antagonistic interactions are widespread in the microbial world and affect microbial evolutionary dynamics. Natural microbial communities often display spatial structure, which affects biological interactions, but much of what we know about microbial antagonism comes from laboratory studies of well-mixed communities. To overcome this limitation, we manipulated two killer strains of the budding yeast Saccharomyces cerevisiae, expressing different toxins, to independently control the rate at which they released their toxins. We developed mathematical models that predict the experimental dynamics of competition between toxin-producing strains in both well-mixed and spatially structured populations. In both situations, we experimentally verified theory's prediction that a stronger antagonist can invade a weaker one only if the initial invading population exceeds a critical frequency or size. Finally, we found that toxin-resistant cells and weaker killers arose in spatially structured competitions between toxin-producing strains, suggesting that adaptive evolution can affect the outcome of microbial antagonism in spatial settings.

The Oleaginous Yeast Metschnikowia pulcherrima Displays Killer Activity against Avian-Derived Pathogenic Bacteria

Simple Summary

Pathogenic bacteria in poultry and the widespread use of antibiotics to manage them are costly in terms of production, environmental risk and human health. Probiotic and other low-cost, non-antibiotic treatments offer attractive alternatives to antibiotic applications, but relatively few of these options exist. In this research, we investigated the potential of an otherwise-useful industrial yeast, Metschnikowia pulcherrima, for the active suppression of poultry pathogenic bacteria. We tested multiple strains of yeast against several important bacterial pathogens and found that the more inhibitory strains of yeast supressed bacterial growth and actively killed the most recalcitrant bacteria. Less aggressive yeast strains could increase the growth of some bacterial strains in some environments. The yeast produced novel molecules in response to the presence of the bacteria and we identified several potential mechanisms by which the yeast inhibited or killed bacteria. Together, these results point towards a useful application of a novel yeast for enhanced, antibiotic-free pathogen control.

Abstract

Metschnikowia pulcherrima is a non-conventional yeast with potential to be used in biotechnological processes, especially those involving low-cost feedstock exploitation and biocontrol applications. The combination of traits that supports these industrial applications in M. pulcherrima also makes it an attractive option to study in the context of livestock health. In this study, we examined the specific interactions between M. pulcherrima and multiple avian pathogenic bacteria. We tested individual bacteria–yeast interactions and bacterial combinations in both solid and liquid media and in variable nutrient environments. Across multiple isolates of M. pulcherrima, we observed different levels of antimicrobial activity, varying from supporting the growth of competing bacteria through suppression and bacterial killing, and we found that these responses varied depending on the bacterial strains and media. We identified multiple molecular routes, including proteins produced by M. pulcherrima strains, that acted to control these microbial interactions. Furthermore, protein screening revealed that M. pulcherrima strains were induced to produce proteins specifically when exposed to bacterial strains, suggesting that fine-tuned mechanisms allow M. pulcherrima to function as a potential lynchpin in a microbial community.

Moniliophthora perniciosa, the Causal Agent of Cacao Witches' Broom Disease Is Killed in vitro by Saccharomyces cerevisiae and Wickerhamomyces anomalus Yeasts

Cacao plantations from South America have been afflicted with the severe fungal disease known as Witches’ Broom Disease (WBD), caused by the basidiomycete Moniliophthora perniciosa. Yeasts are increasingly recognized as good fungal biocides, although their application is still mostly restricted to the postharvest control of plant and fruit decay. Their possible utilization in the field, in a preharvest phase, is nevertheless promising, particularly if the strains are locally adapted and evolved and if they belong to species considered safe for man and the environment. In this work, a group of yeast strains originating from sugarcane-based fermentative processes in Brazil, the cacao-producing country where the disease is most severe, were tested for their ability to antagonize M. perniciosa in vitro. Wickerhamomyces anomalus LBCM1105 and Saccharomyces cerevisiae strains LBCM1112 from spontaneous fermentations used to produce cachaça, and PE2 widely used in Brazil in the industrial production of bioethanol, efficiently antagonized six strains of M. perniciosa, originating from several South American countries. The two fastest growing fungal strains, both originating from Brazil, were further used to assess the mechanisms underlying the yeasts’ antagonism. Yeasts were able to inhibit fungal growth and kill the fungus at three different temperatures, under starvation, at different culture stages, or using an inoculum from old yeast cultures. Moreover, SEM analysis revealed that W. anomalus and S. cerevisiae PE2 cluster and adhere to the hyphae, push their surface, and fuse to them, ultimately draining the cells. This behavior concurs with that classified as necrotrophic parasitism/mycoparasitism. In particular, W. anomalus within the adhered clusters appear to be ligated to each other through roundish groups of fimbriae-like structures filled with bundles of microtubule-sized formations, which appear to close after cells detach, leaving a scar. SEM also revealed the formation of tube-like structures apparently connecting yeast to hypha. This evidence suggests W. anomalus cells form a network of yeast cells connecting with each other and with hyphae, supporting a possible cooperative collective killing and feeding strategy. The present results provide an initial step toward the formulation of a new eco-friendly and effective alternative for controlling cacao WBD using live yeast biocides.

Wickerhamomyces Yeast Killer Toxins' Medical Applications

Possible implications and applications of the yeast killer phenomenon in the fight against infectious diseases are reviewed, with particular reference to some wide-spectrum killer toxins (KTs) produced by Wickerhamomyces anomalus and other related species. A perspective on the applications of these KTs in the medical field is provided considering (1) a direct use of killer strains, in particular in the symbiotic control of arthropod-borne diseases; (2) a direct use of KTs as experimental therapeutic agents; (3) the production, through the idiotypic network, of immunological derivatives of KTs and their use as potential anti-infective therapeutics. Studies on immunological derivatives of KTs in the context of vaccine development are also described.

Yeasts in the attine ant-fungus mutualism: Diversity, functional roles, and putative biotechnological applications

Insects interact with a wide variety of yeasts, often providing a suitable substrate for their growth. Some yeast-insect interactions are tractable models for understanding the relationships between the symbionts. Attine ants are prominent insects in the Neotropics and have performed an ancient fungiculture of mutualistic basidiomycete fungi for more than 55-65 million years. Yeasts gain access to this sophisticated mutualism, prompting diversity, ecological, and biotechnological studies in this environment. We review half a century research in this field, surveying for recurrent yeast taxa and their putative ecological roles in this environment. We found that previous studies mainly covered the yeast diversity from a small fraction of attine ants, being Saccharomycetales, Tremellales, and Trichosporonales as the most frequent yeast or yeast-like orders found. Apiotrichum, Aureobasidium, Candida, Cutaneotrichosporon, Debaryomyces, Meyerozyma, Papiliotrema, Rhodotorula, Trichomonascus, and Trichosporon are the most frequent recovered genera. On the other hand, studies of yeasts' ecological roles on attine ant-fungus mutualism only tapped the tip of the iceberg. Previous established hypotheses in the literature cover the production of lignocellulosic enzymes, chemical detoxification, and fungus garden protection. Some of these roles have parallels in biotechnological processes. In conclusion, the attine ant environment has a hidden potential for studying yeast biodiversity, ecology, and biotechnology, which has been particularly unexplored considering the vast diversity of fungus-growing ants.

Forest Saccharomyces paradoxus are robust to seasonal biotic and abiotic changes

Microorganisms are famous for adapting quickly to new environments. However, most evidence for rapid microbial adaptation comes from laboratory experiments or domesticated environments, and it is unclear how rates of adaptation scale from human-influenced environments to the great diversity of wild microorganisms. We examined potential monthly-scale selective pressures in the model forest yeast Saccharomyces paradoxus. Contrary to expectations of seasonal adaptation, the S. paradoxus population was stable over four seasons in the face of abiotic and biotic environmental changes. While the S. paradoxus population was diverse, including 41 unique genotypes among 192 sampled isolates, there was no correlation between S. paradoxus genotypes and seasonal environments. Consistent with observations from other S. paradoxus populations, the forest population was highly clonal and inbred. This lack of recombination, paired with population stability, implies that selection is not acting on the forest S. paradoxus population on a seasonal timescale. Saccharomyces paradoxus may instead have evolved generalism or phenotypic plasticity with regard to seasonal environmental changes long ago. Similarly, while the forest population included diversity among phenotypes related to intraspecific interference competition, there was no evidence for active coevolution among these phenotypes. At least ten percent of the forest S. paradoxus individuals produced "killer toxins," which kill sensitive Saccharomyces cells, but the presence of a toxin-producing isolate did not predict resistance to the toxin among nearby isolates. How forest yeasts acclimate to changing environments remains an open question, and future studies should investigate the physiological responses that allow microbial cells to cope with environmental fluctuations in their native habitats.

The killer yeast Wickerhamomyces anomalus Cf20 exerts a broad anti-Candida activity through the production of killer toxins and volatile compounds

Candidiasis is a group of opportunistic infections caused by yeast of the genus Candida. The appearance of drug resistance and the adverse effects of current antifungal therapies require the search for new, more efficient therapeutic alternatives. Killer yeasts have aroused as suitable candidates for mining new antifungal compounds. Killer strains secrete antimicrobial proteins named killer toxins, with promissory antifungal activity. Here we found that the killer yeast Wickerhamomyces anomalus Cf20 and its cell-free supernatant (CFS) inhibited six pathogenic strains and one collection strain of Candida spp. The inhibition is mainly mediated by secreted killer toxins and, to a lesser extent, by volatile compounds such as acetic acid and ethyl acetate. A new large killer toxin (>180 kDa) was purified, which exerted 70-74% of the total CFS anti-Candida activity, and the previously described glucanase KTCf20 was inhibitory in a lesser extent as well. In addition, we demonstrated that Cf20 possesses the genes encoding for the β-1,3-glucanases WaExg1 and WaExg2, proteins with extensively studied antifungal activity, particularly WaExg2. Finally, the 10-fold concentrated CFS exerted a high candidacidal effect at 37°C, completely inhibiting the fungal growth, although the nonconcentrated CFS (RCF 1) had very limited fungistatic activity at this temperature. In conclusion, W. anomalus Cf20 produces different low and high molecular weight compounds with anti-Candida activity that could be used to design new therapies for candidiasis and as a source for novel antimicrobial compounds as well.

Honey as an Ecological Reservoir of Antibacterial Compounds Produced by Antagonistic Microbial Interactions in Plant Nectars, Honey and Honey Bee

The fundamental feature of "active honeys" is the presence and concentration of antibacterial compounds. Currently identified compounds and factors have been described in several review papers without broader interpretation or links to the processes for their formation. In this review, we indicate that the dynamic, antagonistic/competitive microbe-microbe and microbe-host interactions are the main source of antibacterial compounds in honey. The microbial colonization of nectar, bees and honey is at the center of these interactions that in consequence produce a range of defence molecules in each of these niches. The products of the microbial interference and exploitive competitions include antimicrobial peptides, antibiotics, surfactants, inhibitors of biofilm formation and quorum sensing. Their accumulation in honey by horizontal transfer might explain honey broad-spectrum, pleiotropic, antibacterial activity. We conclude that honey is an ecological reservoir of antibacterial compounds produced by antagonistic microbial interactions in plant nectars, honey and honey bee. Thus, refocusing research on secondary metabolites resulting from these microbial interactions might lead to discovery of new antibacterial compounds in honey that are target-specific, i.e., acting on specific cellular components or inhibiting the essential cellular function.

Viral RNA Genomes Identified from Marine Macroalgae and a Diatom

Protists provide insights into the diversity and function of RNA viruses in marine systems. Among them, marine macroalgae are good targets for RNA virome analyses because they have a sufficient biomass in nature. However, RNA viruses in macroalgae have not yet been examined in detail, and only partial genome sequences have been reported for the majority of RNA viruses. Therefore, to obtain further insights into the distribution and diversity of RNA viruses associated with marine protists, we herein examined RNA viruses in macroalgae and a diatom. We report the putative complete genome sequences of six novel RNA viruses from two marine macroalgae and one diatom holobiont. Four viruses were not classified into established viral genera or families. Furthermore, a virus classified into Totiviridae showed a genome structure that has not yet been reported in this family. These results suggest that a number of distinct RNA viruses are widespread in a broad range of protists.

A natural approach, the use of killer toxin produced by Metschnikowia pulcherrima in fresh ground beef patties for shelf life extention

A novel killer toxin produced by yeast Metschnikowia pulcherrima was purified and added into ready to cook meatballs to enhance their microbial safety and extension of their shelf life. The agent was added into ready to cook meatballs at two different concentrations (1%-K1 and 2%-K2). The results of those two groups were compared to the control group (K0) lacking the killer toxin. Physical, chemical and microbiological analyses were carried out in meat dough and all analyses were repeated at two day intervals during 10 day-storage at +4 °C. Addition of inhibitor compound in meat dough decreased the numbers of total aerobic mesophillic bacteria, yeast and molds and lactic acid bacteria. Staphylococci/Micrococci, coliform bacteria and total psychrotrophic bacterial counts of the samples were determined as well. Results showed that all indicators of microbial deterioration were found to be higher in K1 group than K2 group, revealing that there was an inverse correlation between the concentration of killer toxin and the number of microorganisms causing spoilage. In addition to 1 log decrease in the number of microorganisms in toxin added groups, the high TBARS values of the control group also showed the effectiveness of the toxin. Toxic effect analysis results showed that the killer toxin had no toxic effect on L929 mouse fibroblast cells after 24h exposure.

Purification and Characterization of WA18, a New Mycocin Produced by Wickerhamomyces anomalus Active in Wine Against Brettanomyces bruxellensis Spoilage Yeasts

Wickerhamomyces anomalus strain 18, isolated from a natural underground cheese ripening pit, secretes a mycocin named WA18 that inhibits wine spoilage yeasts belonging to Brettanomyces bruxellensis species, with a broad-spectrum of activity. WA18 was purified, and the purified protein was digested with specific restriction enzymes (lysine K and arginine R cut sites). The LC-MS and LC-MS/MS analysis after enzymatic digestions revealed a molecular weight of 31 kDa. Bioinformatics processing and database research of digested pure killer protein showed 99% identity with a UDP-glycosyltransferase protein. Competitive inhibition assay of killer activity by cell-wall polysaccharides suggests that branched glucans represent the first receptor site of the toxin on the envelope of the sensitive target. The WA18 partially purified crude extract (PPCE) showed high stability of antimicrobial activity at the physicochemical conditions suitable for the winemaking process. Indeed, in wine WA18 was able to counteract B. bruxellensis and control the production of ethyl phenols. In addition, the strain WA18 was compatible with Saccharomyces cerevisiae in co-culture conditions with a potential application together with commercial starter cultures. These data suggest that WA18 mycocin is a promising biocontrol agent against spoilage yeasts in winemaking, particularly during wine storage.

Killer Yeasts for the Biological Control of Postharvest Fungal Crop Diseases

Every year and all over the world the fungal decay of fresh fruit and vegetables frequently generates substantial economic losses. Synthetic fungicides, traditionally used to efficiently combat the putrefactive agents, emerged, however, as the cause of environmental and human health issues. Given the need to seek for alternatives, several biological approaches were followed, among which those with killer yeasts stand out. Here, after the elaboration of the complex of problems, we explain the hitherto known yeast killer mechanisms and present the implementation of yeasts displaying such phenotype in biocontrol strategies for pre- or postharvest treatments to be aimed at combating postharvest fungal decay in numerous agricultural products.

A Novel Virus Alters Gene Expression and Vacuolar Morphology in Malassezia Cells and Induces a TLR3-Mediated Inflammatory Immune Response

Malassezia is the most dominant fungal genus on the human skin surface and is associated with various skin diseases including dandruff and seborrheic dermatitis. Among Malassezia species, Malassezia restricta is the most widely observed species on the human skin. In the current study, we identified a novel dsRNA virus, named MrV40, in M. restricta and characterized the sequence and structure of the viral genome along with an independent satellite dsRNA viral segment. Moreover, expression of genes involved in ribosomal synthesis and programmed cell death was altered, indicating that virus infection affected the physiology of the fungal host cells. Our data also showed that the viral nucleic acid from MrV40 induces a TLR3-mediated inflammatory immune response in bone marrow-derived dendritic cells, indicating that a viral element likely contributes to the pathogenicity of Malassezia. This is the first study to identify and characterize a novel mycovirus in Malassezia.

Most fungal viruses have been identified in plant pathogens, whereas the presence of viral particles in human-pathogenic fungi is less well studied. In the present study, we observed extrachromosomal double-stranded RNA (dsRNA) segments in various clinical isolates of Malassezia species. Malassezia is the most dominant fungal genus on the human skin surface, and species in this group are considered etiological factors of various skin diseases including dandruff, seborrheic dermatitis, and atopic dermatitis. We identified novel dsRNA segments, and our sequencing results revealed that the virus, named MrV40, belongs to the Totiviridae family and contains an additional satellite dsRNA segment encoding a novel protein. The transcriptome of virus-infected Malassezia restricta cells was compared to that of virus-cured cells, and the results showed that transcripts involved in ribosomal biosynthesis were downregulated and those involved in energy production and programmed cell death were upregulated. Moreover, transmission electron microscopy revealed significantly larger vacuoles in virus-infected M. restricta cells, indicating that MrV40 infection dramatically altered M. restricta physiology. Our analysis also revealed that viral nucleic acid from MrV40 induced a TLR3 (Toll-like receptor 3)-mediated inflammatory immune response in bone marrow-derived dendritic cells, suggesting that a viral element contributes to the pathogenicity of Malassezia.

Antibiotic Activity of Wickerhamomyces anomalus Mycocins on Multidrug-Resistant Acinetobacter baumannii

To evaluate the susceptibility of multidrug-resistant Acinetobacter baumannii to mycocins produced by Wickerhamomyces anomalus and to verify the cytotoxicity of these compounds. Three culture supernatants of W. anomalus (WA40, WA45, and WA92), containing mycocins (WA40M1, WA45M2, and WA92M3), were tested on A. baumannii using broth microdilution methods, solid medium tests, and cytotoxicity tests in human erythrocytes and in Artemia saline Leach. W. anomalus was able to produce high antimicrobial mycocins, as even at high dilutions, they inhibited A. baumannii. In a solid medium, it was possible to observe the inhibition of A. baumannii, caused by the diffusion of mycocins between agar. Finally, the three supernatants were not cytotoxic when tested on human erythrocytes and Artemia salina. According to the evidence in this study, the mycocins of W. anomalus have been effective and could be used in the development of new antimicrobial substances.

Dynamic modelling of the killing mechanism of action by virus-infected yeasts

Killer yeasts are microorganisms, which can produce and secrete proteinaceous toxins, a characteristic gained via infection by a virus. These toxins are able to kill sensitive cells of the same or a related species. From a biotechnological perspective, killer yeasts are beneficial due to their antifungal/antimicrobial activity, but also regarded as problematic for large-scale fermentation processes, whereby those yeasts would kill starter cultures species and lead to stuck fermentations. Here, we propose a mechanistic model of the toxin-binding kinetics pertaining to the killer population coupled with the toxin-induced death kinetics of the sensitive population to study toxic action. The dynamic model captured the transient toxic activity starting from the introduction of killer cells into the culture at the time of inoculation through to induced cell death. The kinetics of K1/K2 activity via its primary pathway of toxicity was 5.5 times faster than its activity at low concentration inducing the apoptotic pathway in sensitive cells. Conversely, we showed that the primary pathway for K28 was approximately three times slower than its equivalent apoptotic pathway, indicating the particular relevance of K28 in biotechnological applications where the toxin concentration is rarely above those limits to trigger the primary pathway of killer activity.

Differential Impacts of Yeasts on Feeding Behavior and Development in Larval Drosophila suzukii (Diptera:Drosophilidae)

Larval Drosophila encounter and feed on a diverse microbial community within fruit. In particular, free-living yeast microbes provide a source of dietary protein critical for development. However, successional changes to the fruit microbial community may alter host quality through impacts on relative protein content or yeast community composition. For many species of Drosophila, fitness benefits from yeast feeding vary between individual yeast species, indicating differences in yeast nutritional quality. To better understand these associations, we evaluated how five species of yeast impacted feeding preference and development in larval Drosophila suzukii. Larvae exhibited a strong attraction to the yeast Hanseniaspora uvarum in pairwise yeast feeding assays. However, larvae also performed most poorly on diets containing H. uvarum, a mismatch in preference and performance that suggests differences in yeast nutritional quality are not the primary factor driving larval feeding behavior. Together, these results demonstrate that yeast plays a critical role in D. suzukii’s ecology and that larvae may have developed specific yeast associations. Further inquiry, including systematic comparisons of Drosophila larval yeast associations more broadly, will be necessary to understand patterns of microbial resource use in larvae of D. suzukii and other frugivorous species.

Purification and molecular characterization of a Metschnikowia saccharicola killer toxin lethal to a crab pathogenic yeast

The marine yeast strain Metschnikowia saccharicola DD21-2, isolated from sediments in the Yalu River, produces a killer toxin with a lethal effect on Metschnikowia bicuspidate strain WCY, a pathogenic yeast strain that infects crabs. In this study, the killer toxin was purified and characterized. After sequential purification, the purity of the killer toxin was increased 72.2-fold over the purity of the yeast cell culture supernatant. The molecular weight of the purified killer toxin was 47.0 kDa. The optimal pH and temperature for killing activity were 5.5°C and 16°C, respectively. The killing activity was stable over a pH range of 4.0-6.5 and temperature range of 0°C-40°C. The purified killer toxin was only effective against toxin-sensitive integral cells and had no killing effect on the protoplasts of toxin-sensitive cells. When exerting the killing effect, the toxin bind to a cell wall receptor of the treated strain, disrupted cell wall integrity and eventually caused death. The amino acid sequence identified by mass spectroscopy indicated that the purified killer toxin might be a protein kinase, but did not show β-1,3-glucanase activity, consistent with the laminarin hydrolysis results. These findings provide a basis for disease prevention and control in marine aquaculture.

Potential of Yeasts as Biocontrol Agents of the Phytopathogen Causing Cacao Witches' Broom Disease: Is Microbial Warfare a Solution?

Plant diseases caused by fungal pathogens are responsible for major crop losses worldwide, with a significant socio-economic impact on the life of millions of people who depend on agriculture-exclusive economy. This is the case of the Witches’ Broom Disease (WBD) affecting cacao plant and fruit in South and Central America. The severity and extent of this disease is prospected to impact the growing global chocolate market in a few decades. WBD is caused by the basidiomycete fungus Moniliophthora perniciosa. The methods used to contain the fungus mainly rely on chemical fungicides, such as copper-based compounds or azoles. Not only are these highly ineffective, but also their utilization is increasingly restricted by the cacao industry, in part because it promotes fungal resistance, in part related to consumers’ health concerns and environmental awareness. Therefore, the disease is being currently tentatively controlled through phytosanitary pruning, although the full removal of infected plant material is impossible and the fungus maintains persistent inoculum in the soil, or using an endophytic fungal parasite of Moniliophthora perniciosa which production is not sustainable. The growth of Moniliophthora perniciosa was reported as being antagonized in vitro by some yeasts, which suggests that they could be used as biological control agents, suppressing the fungus multiplication and containing its spread. Concurrently, some yeast-based products are used in the protection of fruits from postharvest fungal spoilage, and the extension of diverse food products shelf-life. These successful applications suggest that yeasts can be regarded a serious alternative also in the pre-harvest management of WBD and other fungal plant diseases. Yeasts’ GRAS (Generally Recognized as Safe) nature adds to their appropriateness for field application, not raising major ecological concerns as do the present more aggressive approaches. Importantly, mitigating WBD, in a sustainable manner, would predictably have a high socioeconomic impact, contributing to diminish poverty in the cacao-producing rural communities severely affected by the disease. This review discusses the importance/advantages and the challenges that such a strategy would have for WBD containment, and presents the available information on the molecular and cellular mechanisms underlying fungi antagonism by yeasts.

The ecology of killer yeasts: Interference competition in natural habitats

Killer yeasts are ubiquitous in the environment: They have been found in diverse habitats ranging from ocean sediment to decaying cacti to insect bodies and on all continents including Antarctica. However, environmental killer yeasts are poorly studied compared with laboratory and domesticated killer yeasts. Killer yeasts secrete so-called killer toxins that inhibit nearby sensitive yeasts, and the toxins are frequently assumed to be tools for interference competition in diverse yeast communities. The diversity and ubiquity of killer yeasts imply that interference competition is crucial for shaping yeast communities. Additionally, these toxins may have ecological functions beyond use in interference competition. This review introduces readers to killer yeasts in environmental systems, with a focus on what is and is not known about their ecology and evolution. It also explores how results from experimental killer systems in laboratories can be extended to understand how competitive strategies shape yeast communities in nature. Overall, killer yeasts are likely to occur everywhere yeasts are found, and the killer phenotype has the potential to radically shape yeast diversity in nature.

Transcriptome Kinetics of Saccharomyces cerevisiae in Response to Viral Killer Toxin K1

The K1 A/B toxin secreted by virus-infected Saccharomyces cerevisiae strains kills sensitive cells via disturbance of cytoplasmic membrane functions. Despite decades of research, the mechanisms underlying K1 toxicity and immunity have not been elucidated yet. In a novel approach, this study aimed to characterize transcriptome changes in K1-treated sensitive yeast cells in a time-dependent manner. Global transcriptional profiling revealed substantial cellular adaptations in target cells resulting in 1,189 differentially expressed genes in total. Killer toxin K1 induced oxidative, cell wall and hyperosmotic stress responses as well as rapid down-regulation of transcription and translation. Essential pathways regulating energy metabolism were also significantly affected by the toxin. Remarkably, a futile cycle of the osmolytes trehalose and glycogen was identified probably representing a critical feature of K1 intoxication. In silico analysis suggested several transcription factors involved in toxin-triggered signal transduction. The identified transcriptome changes provide valuable hints to illuminate the still unknown molecular events leading to K1 toxicity and immunity implicating an evolutionarily conserved response at least initially counteracting ionophoric toxin action.

Yeast killer toxins: from ecological significance to application

Killer toxins are proteins that are often glycosylated and bind to specific receptors on the surface of their target microorganism, which is then killed through a target-specific mode of action. The killer phenotype is widespread among yeast and about 100 yeast killer species have been described to date. The spectrum of action of the killer toxins they produce targets spoilage and pathogenic microorganisms. Thus, they have potential as natural antimicrobials in food and for biological control of plant pathogens, as well as therapeutic agents against animal and human infections. In spite of this wide range of possible applications, their exploitation on the industrial level is still in its infancy. Here, we initially briefly report on the biodiversity of killer toxins and the ecological significance of their production. Their actual and possible applications in the agro-food industry are discussed, together with recent advances in their heterologous production and the manipulation for development of peptide-based therapeutic agents.

Alternative Methods to SO2 for Microbiological Stabilization of Wine

The use of sulfur dioxide (SO₂ ) as wine additive is able to ensure both antioxidant protection and microbiological stability. In spite of these undeniable advantages, in the last two decades the presence of SO₂ in wine has raised concerns about potential adverse clinical effects in sensitive individuals. The winemaking industry has followed the general trend towards the reduction of SO₂ concentrations in food, by expressing at the same time the need for alternative control methods allowing reduction or even elimination of SO_2. In the light of this, research has been strongly oriented toward the study of alternatives to the use of SO₂ in wine. Most of the studies have focused on methods able to replace the antimicrobial activity of SO₂ . This review article gives a comprehensive overview of the current state-of-the-art about the chemical additives and the innovative physical techniques that have been proposed for this purpose. After a focus on the chemistry and properties of SO₂ in wine_, as well as on wine spoilage and on the conventional methods used for the microbiological stabilization of wine, recent advances on alternative methods proposed to replace the antimicrobial activity of SO₂ in winemaking are presented and discussed. Even though many of the alternatives to SO₂ showed good efficacy, nowadays no other physical technique or additive can deliver the efficacy and broad spectrum of action as SO₂ (both antioxidant and antimicrobial), therefore the alternative methods should be considered a complement to SO₂ in low-sulfite winemaking, rather than being seen as its substitutes.

Unusual relatives of the multisubunit RNA polymerase

Transcription, the first step of gene expression, is accomplished in all domains of life by the multisubunit RNA polymerase (msRNAP). Accordingly, the msRNAP is an ancient enzyme that is ubiquitous across all cellular organisms. Conserved in absolutely all msRNAPs is the catalytic magnesium-binding aspartate triad and the structural fold it is present on, the double ψ β barrel (DPBB). In-depth bioinformatics has begun to reveal a wealth of unusual proteins distantly related to msRNAP, identified due to their possession of the aspartate triad and DPBB folds. Three examples of these novel RNAPs are YonO of the Bacillus subtilis SPβ prophage, non-virion RNAP (nvRNAP) of the B. subtilis AR9 bacteriophage and ORF6 RNAP of the Kluyveromyces lactis cytoplasmic killer system. While YonO and AR9 nvRNAP are both bacteriophage enzymes, they drastically contrast. YonO is an incredibly minimal single-subunit RNAP, while AR9 nvRNAP is multisubunit bearing much more resemblance to the canonical msRNAP. ORF6 RNAP is an intermediate, given it is a single-subunit enzyme with substantial conservation with the msRNAP. Recent findings have begun to shed light on these polymerases, which have the potential to update our understanding of the mechanisms used for transcription and give new insights into the canonical msRNAP and its evolution. This mini-review serves to introduce and outline our current understanding of these three examples of novel, unusual RNAPs.

The Effect of Aspergillus Thermomutatus Chrysovirus 1 on the Biology of Three Aspergillus Species

This study determined the effects of Aspergillus thermomutatus chrysovirus 1 (AthCV1), isolated from Aspergillus thermomutatus, on A. fumigatus, A. nidulans and A. niger. Protoplasts of virus-free isolates of A. fumigatus, A. nidulans and A. niger were transfected with purified AthCV1 particles and the phenotype, growth and sporulation of the isogenic AthCV1-free and AthCV1-infected lines assessed at 20 °C and 37 °C and gene expression data collected at 37 °C. AthCV1-free and AthCV1-infected A. fumigatus produced only conidia at both temperatures but more than ten-fold reduced compared to the AthCV1-infected line. Conidiation was also significantly reduced in infected lines of A. nidulans and A. niger at 37 °C. AthCV1-infected lines of A. thermomutatus and A. nidulans produced large numbers of ascospores at both temperatures, whereas the AthCV1-free line of the former did not produce ascospores. AthCV1-infected lines of all species developed sectoring phenotypes with sclerotia produced in aconidial sectors of A. niger at 37 °C. AthCV1 was detected in 18% of sclerotia produced by AthCV1-infected A. niger and 31% of ascospores from AthCV1-infected A. nidulans. Transcriptome analysis of the naturally AthCV1-infected A. thermomutatus and the three AthCV1-transfected Aspergillus species showed altered gene expression as a result of AthCV1-infection. The results demonstrate that AthCV1 can infect a range of Aspergillus species resulting in reduced sporulation, a potentially useful attribute for a biological control agent.

Unveiling the RNA virosphere associated with marine microorganisms

The study of extracellular DNA viral particles in the ocean is currently one of the most advanced fields of research in viral metagenomic analysis. However, even though the intracellular viruses of marine microorganisms might be the major source of extracellular virus particles in the ocean, the diversity of these intracellular viruses is not well understood. Here, our newly developed method, referred to herein as fragmented and primer ligated dsRNA sequencing (flds) version 2, identified considerable genetic diversity of marine RNA viruses in cell fractions obtained from surface seawater. The RNA virus community appears to cover genome sequences related to more than half of the established positive-sense ssRNA and dsRNA virus families, in addition to a number of unidentified viral lineages, and such diversity had not been previously observed in floating viral particles. In this study, more dsRNA viral contigs were detected in host cells than in extracellular viral particles. This illustrates the magnitude of the previously unknown marine RNA virus population in cell fractions, which has only been partially assessed by cellular metatranscriptomics and not by contemporary viral metagenomic studies. These results reveal the importance of studying cell fractions to illuminate the full spectrum of viral diversity on Earth.

Characterization of two novel mycoviruses from Penicillium digitatum and the related fungicide resistance analysis

Pathogenic fungi including Penicillium digitatum and Penicillium italicum are the main destructive pathogens in the citrus industry, causing great losses during postharvest process. To our knowledge, only one mycovirus from P. digitatum has been reported, and the prevalence of such mycoviruses against citrus postharvest pathogenic fungi and their genotyping were still under investigation. In the present study, we showed that 39 of 152 Penicillium isolates from main citrus-growing areas in China were infected with various mycoviruses belonging to polymycoviruses, Narna-like viruses, and families Totiviridae, Partitivirdae and Chrysoviridae. The next generation sequencing (NGS) towards virus genome library and the following molecular analysis revealed two novel mycoviruses Penicillium digitatum polymycovirus 1 (PdPmV1) and Penicillium digitatum Narna-like virus 1 (PdNLV1), coexisting in P. digitatum strain HS-RH2. The fungicide-resistant P. digitatum strains HS-F6 and HS-E9 coinfected by PdPmV1 and PdNLV1 exhibited obvious reduction in triazole drug prochloraz resistance by mycelial growth analysis on both PDA plates and citrus fruit epidermis with given prochloraz concentration. This report at the first time characterized two novel mycoviruses from P. digitatum and revealed the mycovirus-induced reduction of fungicide resistance.

Edible Films and Coatings as Carriers of Living Microorganisms: A New Strategy Towards Biopreservation and Healthier Foods

Edible films and coatings have been extensively studied in recent years due to their unique properties and advantages over more traditional conservation techniques. Edible films and coatings improve shelf life and food quality, by providing a protective barrier against physical and mechanical damage, and by creating a controlled atmosphere and acting as a semipermeable barrier for gases, vapor, and water. Edible films and coatings are produced using naturally derived materials, such as polysaccharides, proteins, and lipids, or a mixture of these materials. These films and coatings also offer the possibility of incorporating different functional ingredients such as nutraceuticals, antioxidants, antimicrobials, flavoring, and coloring agents. Films and coatings are also able to incorporate living microorganisms. In the last decade, several works reported the incorporation of bacteria to confer probiotic or antimicrobial properties to these films and coatings. The incorporation of probiotic bacteria in films and coatings allows them to reach the consumers' gut in adequate amounts to confer health benefits to the host, thus creating an added value to the food product. Also, other microorganisms, either bacteria or yeast, can be incorporated into edible films in a biocontrol approach to extend the shelf life of food products. The incorporation of yeasts in films and coatings has been suggested primarily for the control of the postharvest disease. This work provides a comprehensive review of the use of edible films and coatings for the incorporation of living microorganisms, aiming at the biopreservation and probiotic ability of food products.

Candidacidal Activity of a Novel Killer Toxin from Wickerhamomyces anomalus against Fluconazole-Susceptible and -Resistant Strains

The isolation and characterization from the sand fly Phlebotomus perniciosus of a Wickerhamomyces anomalus yeast strain (Wa1F1) displaying the killer phenotype was recently reported. In the present work, the killer toxin (KT) produced by Wa1F1 was purified and characterized, and its antimicrobial activity in vitro was investigated against fluconazole- susceptible and -resistant clinical isolates and laboratory strains of Candida albicans and C. glabrata displaying known mutations. Wa1F1-KT showed a differential killing ability against different mutant strains of the same species. The results may be useful for the design of therapeutic molecules based on Wa1F1-KT and the study of yeast resistance mechanisms.

Expression of K1 Toxin Derivatives in Saccharomyces cerevisiae Mimics Treatment with Exogenous Toxin and Provides a Useful Tool for Elucidating K1 Mechanisms of Action and Immunity

Killer toxin K1 is a heterodimeric protein toxin secreted by Saccharomyces cerevisiae strains infected with the M1 double-stranded RNA ‘killer’ virus. After binding to a primary receptor at the level of the cell wall, K1 interacts with its secondary plasma membrane receptor Kre1p, eventually leading to an ionophoric disruption of membrane function. Although it has been under investigation for decades, neither the particular mechanisms leading to toxicity nor those leading to immunity have been elucidated. In this study, we constructed derivatives of the K1α subunit and expressed them in sensitive yeast cells. We show that these derivatives are able to mimic the action of externally applied K1 toxin in terms of growth inhibition and pore formation within the membrane, leading to a suicidal phenotype that could be abolished by co-expression of the toxin precursor, confirming a mechanistic similarity of external and internal toxin action. The derivatives were successfully used to investigate a null mutant completely resistant to externally applied toxin. They provide a valuable tool for the identification of so far unknown gene products involved in K1 toxin action and/or immunity.

Yeast Killer Toxin K28: Biology and Unique Strategy of Host Cell Intoxication and Killing

The initial discovery of killer toxin-secreting brewery strains of Saccharomyces cerevisiae (S. cerevisiae) in the mid-sixties of the last century marked the beginning of intensive research in the yeast virology field. So far, four different S. cerevisiae killer toxins (K28, K1, K2, and Klus), encoded by cytoplasmic inherited double-stranded RNA viruses (dsRNA) of the Totiviridae family, have been identified. Among these, K28 represents the unique example of a yeast viral killer toxin that enters a sensitive cell by receptor-mediated endocytosis to reach its intracellular target(s). This review summarizes and discusses the most recent advances and current knowledge on yeast killer toxin K28, with special emphasis on its endocytosis and intracellular trafficking, pointing towards future directions and open questions in this still timely and fascinating field of killer yeast research.

New Insights into the Genome Organization of Yeast Killer Viruses Based on "Atypical" Killer Strains Characterized by High-Throughput Sequencing

Viral M-dsRNAs encoding yeast killer toxins share similar genomic organization, but no overall sequence identity. The dsRNA full-length sequences of several known M-viruses either have yet to be completed, or they were shorter than estimated by agarose gel electrophoresis. High-throughput sequencing was used to analyze some M-dsRNAs previously sequenced by traditional techniques, and new dsRNAs from atypical killer strains of Saccharomyces cerevisiae and Torulaspora delbrueckii. All dsRNAs expected to be present in a given yeast strain were reliably detected and sequenced, and the previously-known sequences were confirmed. The few discrepancies between viral variants were mostly located around the central poly(A) region. A continuous sequence of the ScV-M2 genome was obtained for the first time. M1 virus was found for the first time in wine yeasts, coexisting with Mbarr-1 virus in T. delbrueckii. Extra 5'- and 3'-sequences were found in all M-genomes. The presence of repeated short sequences in the non-coding 3'-region of most M-genomes indicates that they have a common phylogenetic origin. High identity between amino acid sequences of killer toxins and some unclassified proteins of yeast, bacteria, and wine grapes suggests that killer viruses recruited some sequences from the genome of these organisms, or vice versa, during evolution.

Different Metabolic Pathways Are Involved in Response of Saccharomyces cerevisiae to L-A and M Viruses

Competitive and naturally occurring yeast killer phenotype is governed by coinfection with dsRNA viruses. Long-term relationship between the host cell and viruses appear to be beneficial and co-adaptive; however, the impact of viral dsRNA on the host gene expression has barely been investigated. Here, we determined the transcriptomic profiles of the host Saccharomyces cerevisiae upon the loss of the M-2 dsRNA alone and the M-2 along with the L-A-lus dsRNAs. We provide a comprehensive study based on the high-throughput RNA-Seq data, Gene Ontology and the analysis of the interaction networks. We identified 486 genes differentially expressed after curing yeast cells of the M-2 dsRNA and 715 genes affected by the elimination of both M-2 and L-A-lus dsRNAs. We report that most of the transcriptional responses induced by viral dsRNAs are moderate. Differently expressed genes are related to ribosome biogenesis, mitochondrial functions, stress response, biosynthesis of lipids and amino acids. Our study also provided insight into the virus–host and virus–virus interplays.

Candida pyralidae killer toxin disrupts the cell wall of Brettanomyces bruxellensis in red grape juice

AIMS

The control of the wine spoilage yeast Brettanomyces bruxellensis using biological methods such as killer toxins (instead of the traditional chemical methods, e.g. SO₂ ) has been the focus of several studies within the last decade. Our previous research demonstrated that the killer toxins CpKT1 and CpKT2 isolated from the wine yeast Candida pyralidae were active and stable under winemaking conditions. In this study, we report the possible mode of action of CpKT1 on B. bruxellensis cells in red grape juice.

METHODS AND RESULTS

Brettanomyces bruxellensis cells were exposed to CpKT1 either directly or through co-inoculation with C. pyralidae. This exposure yielded a temporary or permanent decline of the spoilage yeast population depending on the initial cell concentration. Scanning electron microscopy revealed cell surface abrasion while propidium iodide viability staining showed that CpKT1 caused plasma membrane damage on B. bruxellensis cells. Our data show that the exposure to CpKT1 resulted in increased levels of β-glucan, suggesting a compensatory response of the sensitive cells.

CONCLUSIONS

The toxin CpKT1 causes cell membrane and cell wall damage in B. bruxellensis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Candida pyralidae shows potential to be used as a biocontrol agent against B. bruxellensis in grape juice/wine.